SF Bay Area solar system planning: help me avoid making dumb mistakes

[SunEagle]

I can recommend Generac as a very good propane generator. I have one of their 3250w cont - 3750w peak portable units and it does a real nice job. If you want to have a true back up go with a whole house type with a large anchored propane tank.

You may also want to add a second generator using a different fuel type (gas or diesel). That way you will not be depended on just one fuel source for long duration outages.

[Naptown]

Go propane on the fuel for the generator.
Natural gas could be shut down in the event of a broken line due to an earthquake.

[Blammar]

This. The best answer. I never understood the obsession with people wanting to have a dedicated battery system in case of a power outage. Realistically, how often do power outages even occur? If you were to jot down every single power outage for the past 10 years, and calculate whether its worth the additional $30,000-$60,000 for a battery backup system, basic logic and math would dictate that it isn't. If outages are a concern, a cheap, generic generator is a FAR more reasonable and less problematic investment.

Understood. The planning here was to be able to handle the aftereffects of a large earthquake that cut power off to us for two weeks. The number of outages we've had over the past 10 years have been on the order of one a year, for a few hours at most, and that arguably doesn't justify a $30K additional cost. (By the way, how did you get that figure? A 18KWh battery is only about $4k; what am I spending $26k on?)

So Plan B appears to be: go reduce our daily power draw by 50%, look into generators, and decide if solar is actually cost effective at all. (Funny enough, my original plan I had 10 years ago was to purchase a natural gas fuel cell generator... for some reason, they are still not being made.)

[KRenn]

Relax -- my initial belief was that Li Ion batteries were ready for prime time, and had much better life cycle costs than lead acid. I've come to learn otherwise.

Beware of slick salespeople pushing marketing nonsense, its utterly endemic to the solar industry.

[Blammar]

So you want absurdly more toxic and expensive batteries without a great measurable improvement in performance? Why not just go with some AGM's, stable, maintenance free, reasonably priced...etc. It seems more like you are buying into a marketing gimmick then what is actually the best combination of performance and cost-efficiency.

Relax -- my initial belief was that Li Ion batteries were ready for prime time, and had much better life cycle costs than lead acid. I've come to learn otherwise.

[KRenn]

Well your hypothesis is half baked, but not correct. A GTI system is a current source and is required to operate into infinite low resistance of the grid. Your home is an infinite open resistance compared to the grid. If you did not shut down with the grid would likely burn everything electrical in your house up, and be a clear and present danger to utility workers.

To make your wife happy is another $30 to $100K add on option. Plus she has a new part time job requiring 7 days a week no days off job. See if that makes her Happy Happy.

Well now we up the option to $60 to $200K, plus replacement of batteries every 2 or 3 years. That will make you Happy Happy paying for something you rarely if ever use. Kind of like my wife, I never use her much anymore and still paying for it.

Think Grid Tied with whole house generator using diesel, NG, or LPG if you want emergency power.

This. The best answer. I never understood the obsession with people wanting to have a dedicated battery system in case of a power outage. Realistically, how often do power outages even occur? If you were to jot down every single power outage for the past 10 years, and calculate whether its worth the additional $30,000-$60,000 for a battery backup system, basic logic and math would dictate that it isn't. If outages are a concern, a cheap, generic generator is a FAR more reasonable and less problematic investment.

[KRenn]

I'm finding that grid-tied systems will shut down when the grid goes off, even if more than enough power is coming in from the solar panels to energize my house. Clearly, that's unacceptable.

It looks like SolarCity is the only vendor that has the Tesla Li-ion energy storage I want to use (I do not want to use lead acid batteries at all.) So I've put in for some of their free consultation. I'll post here once I get more information.

So you want absurdly more toxic and expensive batteries without a great measurable improvement in performance? Why not just go with some AGM's, stable, maintenance free, reasonably priced...etc. It seems more like you are buying into a marketing gimmick then what is actually the best combination of performance and cost-efficiency.

[Naptown]

Oh. Interesting. My model was that the batteries would be charged via solar during the day then discharged every night. I'm not seeing a time of day option on my power bill, so I get charged the same amount per KWh regardless of when I consume it.

If, on the other hand, I go to a time of day option (assuming it's even available), then I want to use solar during the day as much as possible, and the grid at night. In that model, the batteries behave as you describe.

Okay, if I don't deep discharge the AWG batteries, what am I looking at as their replacement time? 10 years?

What you are actually looking for is a grid tie system with net metering and battery back up.
A TOU plan may help to some extent but for the most part you will be tier shaving (I assume you are on a tiered rate system in Caliland)
I would expect 5 or more years out of the batteries depending on what quality you buy (I generally use Concorde Sunextenders)

[Blammar]

A couple of things
You will not be deep discharging them on a regular basis as they will only be used during the odd outage unless this occurs every day.
Lead acid batteries are almost 100% recyclable and are recycled regularly ( Why do you think the auto parts store charges a core charge if you dont bring in your old battery when you buy a new one. Almost none of a battery ends up in the waste stream.
I do not know about lithium Ion and their ability to be recycled.
$2000 a KWH sounds about right for the Lithium batteries

Oh. Interesting. My model was that the batteries would be charged via solar during the day then discharged every night. I'm not seeing a time of day option on my power bill, so I get charged the same amount per KWh regardless of when I consume it.

If, on the other hand, I go to a time of day option (assuming it's even available), then I want to use solar during the day as much as possible, and the grid at night. In that model, the batteries behave as you describe.

Okay, if I don't deep discharge the AWG batteries, what am I looking at as their replacement time? 10 years?

[Sunking]

Understood, and I can come up with a hypothesis as to why they are designed that way (primarily, there's likely to be a mismatch between what your house requires in power and what the array can deliver.) The "unacceptable" part comes from my wife who says, if we're paying for solar we damn well better have some power to the house when it's sunny and the grid's down.

Well your hypothesis is half baked, but not correct. A GTI system is a current source and is required to operate into infinite low resistance of the grid. Your home is an infinite open resistance compared to the grid. If you did not shut down with the grid would likely burn everything electrical in your house up, and be a clear and present danger to utility workers.

To make your wife happy is another $30 to $100K add on option. Plus she has a new part time job requiring 7 days a week no days off job. See if that makes her Happy Happy.

I was looking at AGM batteries instead, as we're sloppy when it comes to maintenance.

Well now we up the option to $60 to $200K, plus replacement of batteries every 2 or 3 years. That will make you Happy Happy paying for something you rarely if ever use. Kind of like my wife, I never use her much anymore and still paying for it.

Think Grid Tied with whole house generator using diesel, NG, or LPG if you want emergency power.

[Naptown]

Yeah, it does look like there's a premium on Li ion batteries still, sadly. I was hoping that the tech was advancing more rapidly. Sunpower's panels are ~20% efficient; I wasn't able to find any info on Solarcity's panels.

So Plan A is currently a hybrid inverter with a charge controller hooked up to an AGM battery bank. It's still not clear to me what happens when the house needs more power than the inverter can supply. (I'm aware you're not supposed to do that, but I can imagine a couple of years from now I put on a couple more circuits and forget to check the drain vs. the inverter's supply capacity.)

Oh, our house is surrounded by trees, so there'll be shading issues on individual panels during parts of the day. Should I be taking a micro-inverter approach here per panel, which as I understand allows me to get the maximum power that each panel is able to deliver?

You could use microinverters however if you use Enphase and they catch on your warranty is void. This would have to be an AC coupled system at that point so if you run your batteries down past the low voltage cut off you will need a generator to get the system back up by charging your batteries. it gets complicated to do this. Outback has probably the best system for this and how to deal with voltage rise if you are not using all the power from the panels.

[Blammar]

First off there is a difference between kWh and KW. The kWh is the number of kilowatts use per hour. If you used 30kWh/day that comes to an average of 1.25 kilowatts per hour. The 6KW is your peak measurement which is the number of kilowatts used within either a 15 to 30 minute time period depending on your utility. That refers to the highest amount of energy used within a short period of time and it is called your Demand.

Your 1300 to 2000 kWh per month comes to 43 - 66 kWh / day for a 30 day period. If PGE's avg is 15-20kWh/day then you are using at least 3 times that amount and almost 2x the National average.

I would say your first order is to lower your daily use which will save you a lot of money.

Understood. I've ordered a kill-o-watt meter which will arrive tomorrow, and we're going to go to the ac panel and shut off each circuit and check the power use delta (using a stopwatch to time how long it takes the meter to make 1 turn.)

At some point though we do still want to live in the 21st century, so we are still going to fire up our A/C when it gets too hot...

Thanks to everyone so far; I'm learning a lot from this discussion.

[Naptown]

Understood, and I can come up with a hypothesis as to why they are designed that way (primarily, there's likely to be a mismatch between what your house requires in power and what the array can deliver.) The "unacceptable" part comes from my wife who says, if we're paying for solar we damn well better have some power to the house when it's sunny and the grid's down.



I was looking at AGM batteries instead, as we're sloppy when it comes to maintenance. A U.S. Battery bank of 8 AGM L16 batteries is about 18.7KWh and sells for $4.1k or $220/KWh. The Tesla Li-ion packs are expected to cost in the range of $200/KWh (though the current SolarCity prices appear to be around $2000/KWh, which does seem to be price gouging or perhaps just the early-adaptor premium.) The Model S battery is 60KWh with a ~10 year expected lifetime, current replacement cost is $30k, projected replacement cost is $12k.

Okay, I have to agree with you on the prices, though I'm not so sure about the lifetime. Don't AGM batteries have a ~4 year life if you're constantly deep-discharging them?

Regarding toxicity, used lead acid batteries are considered hazardous waste, whereas used li ion batteries are not.

A couple of things
You will not be deep discharging them on a regular basis as they will only be used during the odd outage unless this occurs every day.
Lead acid batteries are almost 100% recyclable and are recycled regularly ( Why do you think the auto parts store charges a core charge if you dont bring in your old battery when you buy a new one. Almost none of a battery ends up in the waste stream.
I do not know about lithium Ion and their ability to be recycled.
$2000 a KWH sounds about right for the Lithium batteries

[Blammar]

Just curious why no lead acid and yet A more complicated and expensive battery. If maintenance is an issue use AGM batteries. They are maintenance free and do not require watering etc.
also you will not be able to get the very high efficiency panels you started with as solar city does not have access to them so you will be able to fit about 2/3 the power on the roof.
The Tesla / Solar city connection is not curious at all as both were founded by the same person.

Yeah, it does look like there's a premium on Li ion batteries still, sadly. I was hoping that the tech was advancing more rapidly. Sunpower's panels are ~20% efficient; I wasn't able to find any info on Solarcity's panels.

So Plan A is currently a hybrid inverter with a charge controller hooked up to an AGM battery bank. It's still not clear to me what happens when the house needs more power than the inverter can supply. (I'm aware you're not supposed to do that, but I can imagine a couple of years from now I put on a couple more circuits and forget to check the drain vs. the inverter's supply capacity.)

Oh, our house is surrounded by trees, so there'll be shading issues on individual panels during parts of the day. Should I be taking a micro-inverter approach here per panel, which as I understand allows me to get the maximum power that each panel is able to deliver?

[SunEagle]

No, my monthly consumption is between 1300 and 2000 KWh per the PGE bill. The 6KW was the peak I believe I measured correctly during a hot summer day.

I'm surprised the national average is 30KWh/day, as PGE is telling me my energy consumption is 4x their average (i.e., PGE is saying their average is closer to 15-20Kwh/day.) You're saying I'm only 2X the national average.

First off there is a difference between kWh and KW. The kWh is the number of kilowatts use per hour. If you used 30kWh/day that comes to an average of 1.25 kilowatts per hour. The 6KW is your peak measurement which is the number of kilowatts used within either a 15 to 30 minute time period depending on your utility. That refers to the highest amount of energy used within a short period of time and it is called your Demand.

Your 1300 to 2000 kWh per month comes to 43 - 66 kWh / day for a 30 day period. If PGE's avg is 15-20kWh/day then you are using at least 3 times that amount and almost 2x the National average.

I would say your first order is to lower your daily use which will save you a lot of money.